BIG PHYSICS, BIG QUESTIONS –

Monster black holes power highest-energy cosmic rays

Supermassive black holes that lie at the centres of galaxies and are devouring their surroundings act as cosmic peashooters, firing energetic charged particles through space (Illustration: NASA)

The Auger Observatory in Argentina detects energetic particles through their interaction with water in surface detector tanks (shown). It also uses telescopes to observe how incoming cosmic rays make the atmosphere fluoresce

(Image: Pierre Auger Observatory)

Enormous black holes in galaxies millions of light years away are pelting us with energetic particles. The finding, from a telescope array 10 times the size of Paris, solves a long-standing mystery about the origins of the most energetic cosmic rays that strike the Earth’s atmosphere.

“Finding an association with something in the sky – it’s just fantastic,” says Alan Watson from the University of Leeds in the UK, a spokesperson for the team that made the discovery. “The result heralds a new window to the nearby universe and the beginning of cosmic-ray astronomy.”

Cosmic rays are charged particles such as protons and atomic nuclei that constantly rain down on Earth’s atmosphere. Most come from the Sun and other sources within our galaxy, such as supernova remnants.

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But the origins of the highest-energy particles, which travel within a whisker of the speed of light, have been puzzling. A single proton can have as much energy as a tennis ball served at 100 kilometres per hour.

Astronomers found it difficult to explain how particles are accelerated to such enormous speeds.

One possibility was that they are spat out by “active galactic nuclei” (AGNs) – energetic galaxies powered by matter swirling onto a supermassive black hole. This would be feasible if the AGNs lay up to a few hundred million light years away. Theory predicts that energetic cosmic rays from farther afield would lose energy before they reached Earth by interacting with relic radiation from the big bang called the cosmic microwave background.

Giant cosmic ray ‘net’

Alternative explanations include gamma-ray bursts, violent explosions that are sometimes thought to signal the collapse of a massive star into a black hole. More bizarrely, the cosmic rays could signal the decay of heavy particlesthat have been trapped inside weird knots in space-time since the big bang.

Testing any idea has been difficult, however, because the highest-energy cosmic rays are very rare. To catch enough of them to investigate their origins, scientists had to build the largest cosmic-ray catcher in the world – the Pierre Auger Observatory.

The international observatory is an array of 1600 detectors covering 3000 square kilometres of land in Argentina. It began operating in 2004 (watch a video overview of the array).

Nearby active galaxies

The observatory measures the aftermath of a cosmic-ray strike. When a high-energy cosmic ray smashes into the upper atmosphere, it creates an avalanche of secondary particles that branches outwards as it cascades down. The particle shower can have a “footprint” as large as 40 square kilometres when it hits the ground.

The ground detectors of the Auger observatory record these secondary particles, while 24 optical telescopes around the perimeter of the array record how the particle shower made the atmosphere fluoresce. This allows astronomers to accurately estimate the energy and trajectory of the incident cosmic ray that caused the shower.

Now the Auger scientists have analysed their 27 most energetic cosmic rays detected until August 2007. Almost all came from directions corresponding roughly to the positions of nearby AGNs up to 250 million light years away from us. The chances of the pattern being a coincidence are just 1 in 100.

“This basically confirms that AGN seem to be the best sources,” says team member Peter Biermann from the Max Planck Institute for Radio Astronomy in Bonn, Germany. “I’m very pleased because this is what I’ve argued on the basis of detailed physics for 20 years.”

Particle smasher

The array’s success ushers in a new era of precision cosmic-ray astronomy, according to Watson. Over time, it should detect tens or hundreds of cosmic rays from individual AGNs and their range of energies should clarify exactly how they were accelerated – a process thought to be controlled by magnetic fields around the colossal black holes.

“If we had, say, 100 cosmic rays from a single AGN, we would see their spectrum of energies and then the models of acceleration would really have something to shoot at,” says Watson.

Scientists will also use the Auger observations to probe the physics of particle collisions. When cosmic rays hit the atmosphere, their collisions can be 30 times more energetic than those at the world’s most powerful particle accelerator, the Large Hadron Collider, which is due to open for business next year in Switzerland.

The Auger observations should also help track magnetic fields in the galaxy, because these fields bend charged cosmic rays slightly off a straight path. “We’re really just starting – we’ve got a fantastic 10 years ahead and it’s really exciting,” says Watson.